A multi-functional fluid is a fluid which can be comprised of several components which can be in different phases, liquid, solid or gaseous. A simple example of a multi-functional fluid is blood. Other multi-functional fluids are, for example, biphasic mixtures consisting of phase change materials, currently called PCMs, in suspension in a liquid and an ice slurry.
In order to be able to resolve the various problems of heat transfer, fluid flow or other, the numerical values of the physical and thermo-physical properties of fluids are of great importance.
Thermal conductivity in particular defines the degree of propagation of heat in a material as a function of the temperature gradient. Conductivity is essentially a transfer of energy under the effect of movement, notably the vibrations of particles. The coefficient of conductivity k (W/m.K) is dependent on the crystalline structure of solids, on the homogeneity, temperature, pressure, of the liquid, solid or gaseous phases and/or the composition.
It is noted that liquids are better conductors than gases, and solids are better conductors than liquids. The conductivity of liquids depends in the first instance on their temperature.
The precise measurement of the coefficient of conductivity is a difficult operation. In fact, the materials which are presently used are not always similar. This leads to differences in the experimental results established by different research laboratories. Thus, the precision related to the coefficient of conductivity does not exceed 5%.
For simple fluids, without a phase change, methods for the measurement of thermal conductivity already exist.
In order to characterize a multi-functional fluid with or without a change of phase, practically no direct, reliable method of measuring thermal conductivity exists.
The German publication DE 199 49 327 A1 describes a method and a device for the implementation of this method for determining the concentration of a gas in a gaseous mixture comprised of several components. The method is based on the measurement of thermal conductivity of a gaseous mixture which is subjected to an increase in temperature between a minimum and maximum value determined by a temperature/time function. An analysis of the curve of temperature variation as a function of time permits the determination of the concentration of a gas contained in the mixture. The device includes a temperature sensor which transmits a signal to a Fourier analyzer. Such a device is not adapted to the measurement of thermal conductivity of a multi-functional fluid.